In connection with timber harvesting, the stumps of the cut trees remain in place in the ground. Tree stumps can also be collected and utilised in energy production or power plants.
A stump to be extracted consists of the cut lowest part of a tree trunk and the roots attached to it, which include some thick roots and numerous thin roots. For example, a fir tree's roots may be spread over a large area around the stump, close to the ground, and a pine tree may in addition have a sturdy main root which has grown downwards under the stump.
The stumps are extracted from the ground, possibly also chopped, and stored to dry in high heaps for a few weeks. After this, they can be hauled to large stockpiles in storage areas for further drying. Stumps require a drying time of 1 to 2 years, after which they are transported to a power plant for crushing and burning. The thermal energy produced is used, for example, for district heat production.
Various stump extraction devices are known. Most often they are devices with clamping parts, jaws or cutting blades. Stump extraction devices are usually suspended on or fixed to the end of the knuckle boom assembly of a moving working machine. The working machine may be an excavator with caterpillar drive, the bucket of which is replaced with the stump extraction device. The stump harvester is controlled by means of the working machine's hydraulic system.
Stump extraction clearly damages the soil and, in addition, the stumps carry impurities, such as soil, sand, gravel and even stones which are attempted to be removed from the stumps by different methods. Cleaning is carried out both during stump extraction and in the storage areas. The impurities cause problems when the stumps are crushed and in the power plants.